This invention relates to wireline logging sondes and to methods of using a logging sonde in a borehole.
It is often necessary to determine parameters such as fluid properties or casing condition or to undertake formation logging in a borehole such as an oilwell borehole. For such purposes a logging sonde is employed which is lowered down the borehole and which has sensors for the required parameter e.g. fluid property or properties. The properties may, for example, include flow rate, dielectric constant, capacitance, resistivity, temperature, thermal capacity, or pH value. It may be required to establish the fluid properties at many points across the borehole, and this is particularly the case with slanting (non-vertical) or horizontal parts of a borehole. Typically therefore a plurality of sensors are employed at different positions across the borehole.
In a borehole three ‘phases’ can be present, namely water, oil and gas. Also, the ambient pressures and temperatures can be extremely high. The individual sensors are contained in pressure-proof capsules. It is necessary to provide electrical connections between the sensors and a telemetry cartridge in the sonde which is connected by wire to the surface. The electrical wires which provide these connections between the sensors and the telemetry cartridge require high pressure seals at the sensors and at the cartridge. These seals are difficult to manufacture and are subject to failure.
U.S. Pat. No. 5,457,988 describes pressure measuring system which uses inductive coupling to connect a downhole well tool to a cable. The cable carries DC to an oscillator adjacent the well tool which inductively transmits power to the tool. Pressure sensors in the tool develop frequency shifted signals to the cable via the inductive coils which provide a digital current modulation which is detected at the earth's surface. The inductive cable connection is essentially a transformer requiring the windings to be located in very close proximity to each other and sharing a common magnetic core. The air gap between the two windings is reduced to the sort of dimension commonly found in conventional signal transformers.
U.S. Pat. No. 4,588,951 describes a mechanical arrangement for urging a measuring pad into engagement in the wall of a borehole. European Patent Application EP-A-0 285 473 describes a mechanical arrangement for increasing the angular field of a logging sonde, and also uses pads.